Filtering devices—called piston type screen changers are known for filtering high-viscosity media such as plastic melts. The filtering device has a housing and feed channels. Two pistons are arranged movably in the housing. Each of these pistons has at least one cavity. At least one filter element or screen is placed in each cavity. In the production mode of operation, the feed channels in the housing branch off towards the cavities in such a way that melt is guided through the respective filter elements. Provided behind the filter element, when viewed in the direction of flow, are partial channels through which the plastic melt is discharged. The partial channels are combined at some point in the housing.
The filtering device with two pistons bearing the filter elements or screens and being movable in a direction basically perpendicular to the flow-through direction of the fluid has several advantages. One of these is that the piston is not only a filter carrier but closes and opens flow paths with its movement so that no additional valves are needed. By a suitable design of flow channels the piston can be moved from a production mode of operation into a back-flushing mode of operation. In the back-flushing mode of operation the fluid is led from the outlet side in a reverse flow direction to the screen, in order to detach impurities stuck to the screen and to flush them out of the housing. Furthermore the piston can be partially moved out of the housing in order to give access to a filter element when cleaning or replacement thereof has become necessary. The screen on the other piston is not adversely affected by the back-flushing process. Thus the filtering device remains operational with at least 50% of the screens.
DE 10 2005 043 096 A1 discloses a filtering device having two piston. Each piston has two filter cavities. While the screen in one cavity is cleaned by back-flushing or is replaced, not only the screens on the other pistons but also the second screen on the same piston remains operational whereby 75% of the total screen area in the filtering device is still used during cleaning or replacement of one screen.
EP 554 237 A1 discloses a filtering device with two pistons, each piston having at least one cavity. Each cavity having a filter element or screen which can be back-flushed by means of an external unit having a collective reservoir and a hydraulically driven single displacing piston disposed therein. By retracting the displacing piston fluid is collected in the reservoir. By pushing the piston forward the pressure in all downstream flow channels is increased in order to improve the cleaning effect during back-flushing. The drawback of this device is that the increased fluid pressure does not only effect the screens in the cavities of the pistons but also effects other equipment in the other direction of the flow channel such as an extrusion die which might be adversely affected by the increased fluid pressure.
In EP 1778 379 B1 an improved filtering device is disclosed having an additional displacing piston for each screen cavity which is used for back-flushing. The displacing piston is inserted into a partial outlet channel which leads to one cavity. The displacing piston presses clean fluid, in reverse flow, from the backside through the screen. The effective pressure results from the force of the displacing piston thus being independent from the fluid pressure in the system.
DE 10 2011 001 262 A1 shows a further improvement of such a filtering device. The displacing piston has an internal channel which, in the production mode, is part of the partial outlet channel extending from an inlet opening in the front face of the displacing piston to an outlet opening at the side wall. Once the displacing piston is retracted the flow path is interrupted and simultaneously located in front of the piston, a reservoir with a larger diameter, is filled with fluid coming through the screen. For back-flushing the displacing piston is moved forward in order to push out the fluid portion stored in the reservoir in a reverse flow direction through the screen.
The known filtering devices with a separate displacing piston per cavity achieve very good cleaning results without adverse impact on the filtering process, which is still going on through the remaining cavities. However, having many displacing piston units results in high complexity of both mechanical as well as electrical design.
The problem addressed by the present invention is to improve a filtering device of the aforementioned kind.